1. FIELD OF THE INVENTION
The present invention relates to a semiconductor device whose characteristics are scarcely deteriorated by ionizing radiations such as .gamma.-rays, X-rays, and an electron beam.
2. DESCRIPTION OF THE PRIOR ART
When a conventional bipolar silicon transistor, for example, a lateral bipolar silicon transistor which includes an emitter 4, a base 3, a collector 5, and an oxide film 1 kept in contact with the surface of an active base region, in a manner such as shown in FIG. 1, is irradiated with an ionizing radiation 100, a positive charge 10 is formed in the oxide film 1, and a surface state 20 is formed at an interface 2 between the oxide film and silicon. Reference numerals 4' and 5' in FIG. 1 respectively represent emitter and collector electrodes. The number of surface states increases exponentially with the total dose of the ionizing radiation 100. The surface state 20 acts as the recombination center, and therefore increases the surface recombination rate of minority carriers. Accordingly, when an ionizing radiation is incident upon a lateral transistor such as shown in FIG. 2 (in FIG. 2, reference numerals 3', 4' and 5' represent base, emitter and collector electrodes, respectively, and 40 and 50 represent the semiconductor substrate and an isolating region, both of p-conductivity), the current amplification factor h.sub.FE of the transistor indicated by a curve 300 in FIG. 3 becomes far smaller, as compared with the current amplification factor at a time before the transistor is exposed to the radiation (namely, the current amplification factor indicated by a curve 200 in FIG. 3). Specifically, the reduction in current amplification factor due to the ionizing radiation is remarkable in a low current region. This is because a surface recombination current is increased by exposing the transistor to the ionizing radiation. Now, the surface recombination current will be explained below in detail.
FIG. 4 shows a fundamental pn junction part of a bipolar element. Referring now in particular to the drawings, wherein like reference characters designate like or corresponding parts throughout, there is shown a depletion layer designated by reference numeral 400. When the number of surface states 20 caused by an ionizing radiation 100 is increased, the surface recombination rate of minority carriers is also increased. In more detail, the surface recombination rate is approximately proportional to the number of surface states. Further, the surface recombination current is proportional to the above surface recombination rate of minority carriers. Therefore, the surface recombination current increases with the radiation dose. Further, the surface recombination occurs not only in a region between an emitter 4 and a collector 5 but also in a peripheral portion of each of the emitter 4 and collector 5, and such recombinations also should be prevented.
FIG. 5 shows relations between base-emitter voltage V.sub.BE and collector current I.sub.C or base current I.sub.B at an actual lateral transistor, which were experimentally obtained for various values of total radiation dose. The current amplification factor h.sub.FE is defined by a ratio of the collector current I.sub.C to the base current I.sub.B. As can be seen from FIG. 5, the collector current I.sub.C is scarcely affected by the radiation. That is, the above experimental results show that a reduction in current amplification h.sub.FE caused by the radiation is based upon an increase in base current I.sub.B. The increase in base current is considered to be caused by an increase in surface recombination current.
FIG. 6 shows a surface recombination current obtained by a two-dimensional device simulation using a computer. In FIG. 6, the Y-axis indicates a surface recombination current density in an arbitrary scale, and the X-axis a distance along the surface of a lateral pnp transistor shown in the upper righthand corner of FIG. 6. As shown in FIG. 6, not only the surface of a depletion layer but also the surface of an n-base layer is greatly affected by the radiation. In more detail, a pnp transistor and a npn transistor have a drawback that a portion of base layer kept in contact with an oxide film is affected by a radiation and thus the characteristics of the transistor are deteriorated by the radiation.